This invention relates to improvements in processes of producing semiconductor planar devices such as semiconductor planar diodes, planar transistors, planar thyristors, planar bidirectional triode thyristors such as triac (trade mark).
Semiconductor planar devices include the PN junction inevitably intersecting the main face of the semiconductor substrate and therefore having its curved surface portion. An electric field involved tends to be concentrated in the curved surface portion of the junction so that the curved surface portion of the junction may be broken down with a lower applied voltage than that a breakdown voltage the flat portion thereof. In that event the smaller the radius of curvature of the curved surface portion of the PN junction the less the breakdown voltage will be. In order to produce high breakdown voltage semiconductor planar devices, it is required to effect the deep diffusion to increase the radius of curvature. In planar thyristor etc. this deep diffusion causes both the current gain and cut-off frequency to be decreased.
Also electrically insulating films for coating the PN junction have been previously formed of silicon dioxide (SiO.sub.2) prepared either by thermally oxidizing the surface of silicon substrates or by chemically processing the surface of silicon substrates followed by the growth from gaseous phase. As well known, a positive surface charge is present at the interface between the substrate surface and the silicon dioxide film thus formed. This positive surface charge restrains a depletion layer for the PN junction from spreading in the vicinity of the substrates surface. Thus the presence of the positive surface charge in an excessive quantity causes the particular electric field to be concentrated in the vicinity of the surface of the substrate resulting in the occurrence of the breakdown with a low voltage.
Therefore it has been heretofore difficult to impart the high breakdown voltage characteristic to semiconductor planar devices.